Known designs all have defects or deficiencies in one or more of these areas and it would therefore be desirable to provide a klystron amplifier embodiments of which can be designed to meet the above criteria.
According to a first aspect of the present invention there is provided a klystron amplifier comprising means defining a plurality of electron beam paths and means defining plural damped disc-shaped cavities, wherein the plurality of electron beam paths cut the cavities and the Klystron amplifier further comprises an annular input cavity and an annular output cavity disposed around the substantially circular external periphery of respective disc-shaped cavities in communication therewith, the output cavity is arranged to receive RF power from the electron beams, wherein the cavities are arranged to support one of a single resonant rotating wave in a whispering-gallery mode, and a single resonant standing wave in a whispering-gallery mode.